StandardPreconditioners_gpu_serial.hpp

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/*
  Copyright 2025 Equinor ASA
 
  This file is part of the Open Porous Media project (OPM).
  OPM is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
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  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
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*/
 
#ifndef OPM_STANDARDPRECONDITIONERS_GPU_SERIAL_HEADER
#define OPM_STANDARDPRECONDITIONERS_GPU_SERIAL_HEADER
 
 
#include <dune/istl/bcrsmatrix.hh>
#include <type_traits>
 
 
namespace Opm
{
 
 
template <class Operator>
struct StandardPreconditioners<Operator,
                               Dune::Amg::SequentialInformation,
                               typename std::enable_if_t<Opm::is_gpu_operator_v<Operator>>> 
{
 
    using O = Operator;
    using C = Dune::Amg::SequentialInformation;
    using F = PreconditionerFactory<O, C>;
    using M = typename F::Matrix;
    using V = typename F::Vector;
    using P = PropertyTree;
    using PrecPtr = typename F::PrecPtr;
 
    using field_type = typename V::field_type;
 
    static constexpr int maxblocksize = 6;
    static void add()
    {
        F::addCreator("ILU0", [](const O& op, const P& prm, const std::function<V()>&, std::size_t) {
            const double w = prm.get<double>("relaxation", 1.0);
            using GpuILU0 =
                typename gpuistl::GpuSeqILU0<M, gpuistl::GpuVector<field_type>, gpuistl::GpuVector<field_type>>;
            return std::make_shared<GpuILU0>(op.getmat(), w);
        });
 
        F::addCreator("JAC", [](const O& op, const P& prm, const std::function<V()>&, std::size_t) {
            const double w = prm.get<double>("relaxation", 1.0);
            using GPUJac = typename gpuistl::GpuJac<M, gpuistl::GpuVector<field_type>, gpuistl::GpuVector<field_type>>;
            return std::make_shared<GPUJac>(op.getmat(), w);
        });
 
        // The next two (OPMILU0 and DILU) are the GPU preconditioners that need CPU matrices.
        // Since we are not storing the block size compile time for the GPU matrices 
        // **and** we need the CPU matrix for the creation, we need to
        // dispatch the creation of the preconditioner based on the block size. 
        //
        // Note that this dispatching is not needed in the future, since we will have a constructor taking GPU matrices directly.
        F::addCreator("OPMILU0", [](const O& op, [[maybe_unused]] const P& prm, const std::function<V()>&, std::size_t) {
            return dispatchOnBlockSize<maxblocksize>(op, [&](const auto& cpuMatrix) {
                const bool split_matrix = prm.get<bool>("split_matrix", true);
                const bool tune_gpu_kernels = prm.get<bool>("tune_gpu_kernels", true);
                const int mixed_precision_scheme = prm.get<int>("mixed_precision_scheme", 0);
                using CPUMatrixType = std::remove_const_t<std::remove_reference_t<decltype(cpuMatrix)>>;
                using GPUILU0 =
                    typename gpuistl::OpmGpuILU0<CPUMatrixType, gpuistl::GpuVector<field_type>, gpuistl::GpuVector<field_type>>;
 
                return std::make_shared<GPUILU0>(op.getmat(), cpuMatrix, split_matrix, tune_gpu_kernels, mixed_precision_scheme);
            });
        });
 
        F::addCreator("DILU", [](const O& op, [[maybe_unused]] const P& prm, const std::function<V()>&, std::size_t) {
            return dispatchOnBlockSize<maxblocksize>(op, [&](const auto& cpuMatrix) {
                const bool split_matrix = prm.get<bool>("split_matrix", true);
                const bool tune_gpu_kernels = prm.get<bool>("tune_gpu_kernels", true);
                const int mixed_precision_scheme = prm.get<int>("mixed_precision_scheme", 0);
                using CPUMatrixType = std::remove_const_t<std::remove_reference_t<decltype(cpuMatrix)>>;
                using GPUDILU =
                    typename gpuistl::GpuDILU<CPUMatrixType, gpuistl::GpuVector<field_type>, gpuistl::GpuVector<field_type>>;
                return std::make_shared<GPUDILU>(op.getmat(), cpuMatrix, split_matrix, tune_gpu_kernels, mixed_precision_scheme);
            });
        });
    }
 
private:
 
    template<class BlockType>
    static Dune::BCRSMatrix<BlockType> makeCPUMatrix(const O& op) {
        // TODO: Make this more efficient. Maybe we can simply copy the memory areas directly?
        //       Do note that this function is anyway going away when we have a GPU
        //       constructor for the preconditioners, so it is not a priority.
        const auto& gpuMatrix = op.getmat();
 
        const auto nonZeros = gpuMatrix.getNonZeroValues().asStdVector();
        const auto rowIndices = gpuMatrix.getRowIndices().asStdVector();
        const auto columnIndices = gpuMatrix.getColumnIndices().asStdVector();
 
        const auto numberOfNonZeroes = gpuMatrix.nonzeroes();
        const auto N = gpuMatrix.N();
 
        Dune::BCRSMatrix<BlockType> matrix(N, N, numberOfNonZeroes, Dune::BCRSMatrix<BlockType>::row_wise);
        for (auto row = matrix.createbegin(); row != matrix.createend(); ++row) {
 
            for (auto j = rowIndices[row.index()]; j != rowIndices[row.index() + 1]; ++j) {
                const auto columnIndex = columnIndices[j];
                row.insert(columnIndex);
            }
        }
 
        for (std::size_t i = 0; i < N; ++i) {
            for (auto j = rowIndices[i]; j != rowIndices[i + 1]; ++j) {
                const auto columnIndex = columnIndices[j];
                // Now it gets a bit tricky, first we need to fetch the block matrix
                BlockType blockMatrix;
                constexpr static auto rows = BlockType::rows;
 
                for (std::size_t k = 0; k < rows; ++k) {
                    for (std::size_t l = 0; l < rows; ++l) {
                        blockMatrix[k][l] = nonZeros[j * rows * rows + k * rows + l];
                    }
                }
                matrix[i][columnIndex] = blockMatrix;
            }
        }
 
        return matrix;
    }
 
    template<int blocksizeCompileTime, class CreateType>
    static PrecPtr dispatchOnBlockSize(const O& op, CreateType create) {
        if (op.getmat().blockSize() == blocksizeCompileTime) {
            const auto cpuMatrix = makeCPUMatrix<Dune::FieldMatrix<field_type, blocksizeCompileTime, blocksizeCompileTime>>(op);
            return create(cpuMatrix);
        } 
        if constexpr (blocksizeCompileTime > 1) {
            return dispatchOnBlockSize<blocksizeCompileTime - 1>(op, create);
        }
        else {
            throw std::runtime_error(fmt::format("Unsupported block size: {}. Max blocksize supported is {}.", op.getmat().blockSize(), maxblocksize));
        }
    }
    
};
 
 
} // namespace Opm
 
#endif // OPM_STANDARDPRECONDITIONERS_GPU_SERIAL_HEADER